This invention relates to a device and a method for reducing the size of a color image to display various color images simultaneously on a screen capable of displaying several colors.
A color gamut for a typical color monitor consists of approximately 16 million colors, all of which are not required in a single picture in many color applications. The range of colors that can be displayed on the monitor is clearly smaller than that of all colors. For example, some color systems have a color monitor allowing 256 colors to be displayed simultaneously. Such a color system often includes a look-up table having as many entries as there are pixel values. Each pixel of a pixel map (color image) has any one of the pixel values as an index into the look-up table. The look-up table stores the table entries and the corresponding red(R), green(G), and blue(B) component values. To display a color image on a screen, the table entry is accessed for each pixel, thereby loading the RGB component values. The color system also comprises a single look-up table where the loaded RGB component values are stored. The RGB component values are used to control the color monitor for displaying color images with, for example, 256 colors.
In such a conventional color system, a different look-up table is required for every one original color image and thus a plurality of look-up tables are necessary for displaying various color images. To display a plurality of color images simultaneously on a single screen, each original image must be reduced in size while the corresponding look-up table is being accessed. Upon displaying the reduced images on the screen, the look-up operation is done for each reduced image with its corresponding look-up table. Color or chromlnal information for a first reduced image is loaded up from a first look-up table, following which the first image is displayed in a given region on the screen using the look-up table in the system. Subsequently, color information for a second reduced image is loaded up from a second look-up table. The look-up table in the system is again used to display the second image on the screen. This may result in a problem such as the appearance of the second image causing the first image to disappear from the screen. With respect to this, a common look-up table (hereinafter referred to as a reference look-up table) is prepared in which the RGB values are assigned previously to the table entries. This RGB value is referred hereinafter to a reference RGB value as distinguished from the RGB value stored in the "individual" look-up tables.
An original color image is divided into several regions or partitions and thus the resulting image consists of m by n regions, where m and n are predetermined positive integers larger than one. Each region includes a plurality of pixels composing the original color image. The RGB value of each pixel is obtained for every region by accessing the look-up table based on the pixel values. Subsequently, an average is calculated for the RGB values of all pixels in a single region to determine a representative pixel having the RGB value (hereinafter referred to as a representative RGB value) equal to the average RGB value. In this way, one representative pixel is obtained for each region, that is, a reduced color image consists of the representative pixels equal in number to the regions. The reference look-up table is then accessed to obtain the reference RGB value that is closest to the representative RGB value of each representative pixel. In addition, the table entry of the representative pixel is translated into the table entry of its closest reference RGB value. This translation between the table entries is carried out for all look-up tables. As a result, the table entries of each look-up table are translated into the table entries of the reference look-up table that, in turn, corresponds to the look-up table in the system. The table entry of the reference look-up table is referred to as a reference look-up table hereinafter. The desired color images can thus be displayed simultaneously on the screen by means of accessing the reference look-up table according to the pixel values of the representative pixels. This could be the clue to search the images.
The above mentioned conventional method requires that the look-up table be loaded as many times as the number of the representative pixels corresponding to the number of translation of the table entries. This means that the original image having screen resolution of 400 by 200 pixels can be reduced into an image including the representative pixels of 200 by 100 after the translation of twenty thousand times. Such a time-consuming processing is undesirable by various considerations.
Accordingly, the present invention is directed to overcome the above problems and an object thereof is to provide a device and a method for reducing the size of a color image at high speed.